how we can encourage disease microorganisms to evolve away from virulence

[Nancy]

Recently posted talk from the 07 TED conference: "Using Evolution to Design Disease Organisms Intelligently"

http://www.ted.com/index.php/talks/view/id/259

Microbiologist Paul Ewald explains how we can intervene to encourage, even force disease microorganisms to evolve away from virulence and toward a form that causes milder disease.

This is a lively 18-minute video which focuses primarily on managing cholera and malaria, and more briefly tuberculosis and AIDS.

[gsgs]

Re: how we can encourage disease microorganisms to evolve away from virulence

influenza should be a good example. We know, that different influenza
strains compete via host-immunity and in pandemics one strain
wiped out another one.
We had earlier discussed the idea of a deliberate "counter pandemic" with
another strain, so to cause immunity against the more virulent strain.

Influenza evolution is rather mysterious, we don't know which strain will be prevalent the next season. Once we understand this better we might
try to influence it...

[Nancy]

Re: how we can encourage disease microorganisms to evolve away from virulence

The concept Ewald is building on is that micro-organisms are more likely to evolve toward greater virulence if they have a method for reaching a new host even when they have made the current host too sick to move. Likewise, they will evolve toward mildness if they are denied that intermediate medium or vector and forced to find new hosts by direct human-to-human contact.

Virulent cholera, for example, can easily spread through drinking water when contaminated water used to wash patients is thrown into the water supply. It does not need direct person-to-person contact.

Likewise, it doesn't matter that the malaria patient is too sick to move. The parasite-bearing mosquito will happily come to the patient - unless all the windows are screened. If the windows are screened, the patient needs to be able to walk out the door to get bitten, and those malaria parasites that make a patient too sick to walk outdoors will die with their victim.

Ewald's idea is that we can use environmental changes to encourage changes in the "germ." Cleaning up the water supply not only provides clean water and fewer opportunities for cholera in general, it also selects against the more virulent forms of the disease that immobilize the victim. In the same way, mosquito nets and window screens not only reduce the number of mosquitoes that can reach the sick person, but also create an environment more favorable for the spread of milder forms of the disease than the more virulent ones.

[gsgs]

Re: how we can encourage disease microorganisms to evolve away from virulence

thanks for the summary. He spoke so quickly

this evolution towards milder strains only makes sense
if there is competition, if several cholera or Malaria strains
would compete and kill each other. Is this the case ?
I can see the evidence for influenza.

Else you may just have virulent CholeraA and not so virulent
CholeraB independently of each other and of course, you
would concentrate your forces on the more virulent one,
but the non-virulent one doesn't additionally profit
and then "help" in the fight.
(so better kill them both)

[Nancy]

Re: how we can encourage disease microorganisms to evolve away from virulence

Yes, he DID speak fast! Ted speakers are required to limit their talks to 18 minutes, which means they have to explain their essential points very quickly.

While this short format tends to produce exciting, often exhilarating talks, it does not allow speakers to lay out a complete "proof" of their ideas. Interested audience members are expected to follow up by reading the speakers' books or articles.

Not being a scientist, I can't assess the strengths and weaknesses of Ewald's argument. I just recognized that it would be of interest to FT readers.

[tropical]

Re: how we can encourage disease microorganisms to evolve away from virulence

I don't want to enter the possible new med. frontiers, but from other "innovative" human technologies, historicaly learned, their's bad collateral outcomes was seen only later.
___
A little sf bookreader break:

That story remembers the resolving tryings in the sf book "... grass", where the virulent grass virus named "Chung Lee(~or similar)" was attacked by an sintetic human made counter part virus (designed to not harm grass) which was aspected to prevail on the natural one.

At first (a season) that seems working.
The next one, starts to be visible that the sintetic capitulate.
But now, there were (4?) more versions of the natural virulent virus, which starts to spreading further.

As wroted in the story, if the sintetic virus had a strenght and a capacity of a thousend year of duration like the R. empire it would prevail, but it was like a ... empire, short living in nature.
___

What would be this designed one: a R. empire, or ...

Additionaly from researchs where is included the sci. 6th man on the moon, seems that evolution "learns".

[Allan]

Re: how we can encourage disease microorganisms to evolve away from virulence

thanks for the summary. He spoke so quickly

this evolution towards milder strains only makes sense
if there is competition, if several cholera or Malaria strains
would compete and kill each other. Is this the case ?
I can see the evidence for influenza.

Else you may just have virulent CholeraA and not so virulent
CholeraB independently of each other and of course, you
would concentrate your forces on the more virulent one,
but the non-virulent one doesn't additionally profit
and then "help" in the fight.
(so better kill them both)

It is not necessary for distinct strains to exist and compete. It is only necessary for there to be genetic variation in virulence. Such genetic variation will permit shifting of virulence in whatever direction is made appropriate by environmental factors which affect the reproductive advantages or liabilities, to the disease agent, of virulence.

The main point here is that it is not necessary for two strains to compete - the change in virulence can happen within a single strain in response to environmental influences, including the sorts mentioned by the author.

AE

[gsgs]

Re: how we can encourage disease microorganisms to evolve away from virulence

Maybe I don't understand.

But when the viruses don't compete, then they just evolve independently.
Growth of one does not influence the other.
One will be more successful, but you can't reduce or eliminate either one
by growing the other.

You can get a change in virulence by artificially introducing another
less virulent virus. But don't expect this will reduce the virulent one.
You just get a nonvirulent one on top of, in addition to the virulent one.
Then the nonvirulent one is prevalent and you claim for a change
in virulence, although the virulent one isn't reduced ?

Fighting flu with Rhinoviruses will reduce the average virulence of respiratory
diseases, but won't reduce the flu.

[Allan]

Re: how we can encourage disease microorganisms to evolve away from virulence

Maybe I don't understand.

But when the viruses don't compete, then they just evolve independently.
Growth of one does not influence the other.
One will be more successful, but you can't reduce or eliminate either one
by growing the other.

You can get a change in virulence by artificially introducing another
less virulent virus. But don't expect this will reduce the virulent one.
You just get a nonvirulent one on top of, in addition to the virulent one.
Then the nonvirulent one is prevalent and you claim for a change
in virulence, although the virulent one isn't reduced ?

Fighting flu with Rhinoviruses will reduce the average virulence of respiratory
diseases, but won't reduce the flu.

Perhaps this is semantics, but my suggestion is that the competition occurs among individual virions, not between pre-existing, separate strains. Since virulence varies genetically (I am assuming) among the individual reproducing virions, some will do better under the environmental (from a virus's point of view) conditions experienced. The individual virions whose genetically determined virulence is advantageous will produce more "offspring" (ie. by infecting more individuals, or spreading more quickly etc.) than those individuals whose genetically determined virulence is less optimal. Over time (for viruses, presumably a fairly short time as evolution goes), the more optimal virulence will increase in the viral population, and less optimal virulence will decline.

My suggestion is that this process does not depend on the existence of two or more competing "strains", it only requires that the virulence of individual virions varies genetically. I think that it also requires that immune response of the host overlap among the virions which are competing, i.e. that if the virion with the most optimal virulence gets to a host first, the other virulence types it is competing with will either not be able to infect the host, or will do so less effectively. In this way, the more optimal virulence can over time displace less optimal ones.

Without some degree of immune response overlap between virulence "types", I think your point regarding separate evolution of strains would be correct., i.e. that they would just do their own thing without affecting one another. I am no expert here, but I believe that immune response overlap of some degree would be more the norm than otherwise between closely related influenza A viruses.